Create new prom metric for stx blocks in last btc block

[CharlieC3]

To help improve our internal alerting systems, we need to be able to determine how many stacks blocks were generated as part of the previous bitcoin block. If we can bubble this up, we'd potentially be able to alert on issues much faster.

[rafaelcr]

@CharlieC3 can you clarify, do you mean "how many Stacks blocks per previous Bitcoin block" ? Or "how many Stacks blocks per previous tenure"? (Keeping in mind the two are not always equivalent if there's a missed sortition)

[CharlieC3]

@rafaelcr That's a good point and to be honest there might be usefulness in having both of them if it doesn't complicate the effort too much.

[rafaelcr]

@CharlieC3 I'm not sure if we get a /new_block event in case of a missed sortition but I will investigate